I ask because the sub-frames under that frame which is labelled 'a' seem to have slightly wonky dimensions when playing it from the .swf file. I was wondering if this mysterious letter a was the cause of it, or if the weird dimensions were due to something else.

On the stage I can tell that the sub-frames look fine due to comparing with the other frames by flicking back and forth, but when ctrl + enter is pressed, certain sections become abnormally stretched out. Thus I deduced that I might have accidentally added a filter or something and wondered if this 'a' marker was an indication of it.

Also, does anyone know if this 'a' thing exists in ActionScript 3? Is Actionscript 3 better than 1 and 2?

The lower case "a" is simply indicating that the Actions panel has something in it for that frame. I would say it means that there is code in that frame but that is not necessarily the case. It can also mean there is anything in the Actions panel for that frame, such as if you went into the panel and just hit a carriage return or the spacebar.

This is not related to Actionscript versions, just the Actions panel versus the timeline... it happens with any version of Flash / Actionscript.

AS3 is better than AS1 and AS2, AS2 is better than AS1. AS3 is faster and more current as an object oriented language - which helps keep your skillset up to date with an everchanging world. Whether or not that is meaningful depends on the kind of work you do. You can still create perfectly nice stuff using AS2. It's just that some things are easier to accomplish using AS3 (and sometimes vice versa, but less often)